The operation of many electronic and mechanical devices is dependent upon the physical location or position of another device. For example, the speed of a vehicle is generally controlled by an accelerator, the output of which depends upon the linear position of an accelerator pedal. In traditional acceleration control mechanisms, the pedal is connected electrically, mechanically, or both, to an accelerator controller. As the vehicle operator moves (e.g., presses or releases) the pedal, the accelerator increases or decreases the speed of the vehicle in a manner directly related to the movement. In pursuit of safety and ease of use, designers in the art have sought to develop acceleration mechanisms which control the speed of the vehicle with a high degree of precision in relation to the location of the pedal.
FIGS. 1A and 1B show one of the most primitive, yet most widely used, acceleration control mechanisms. In FIG. 1A, an accelerator pedal 22 has a leg 20 which mechanically contacts the wiper shaft 24 of a multi-turn potentiometer 26. As the pedal 22 is depressed or released, the motion of the leg 20 causes the wiper shaft 24 to rotate. As shown in FIG. 1B, when the wiper shaft 24 rotates, the wiper 28 of the potentiometer 26 moves between the ends 30a and 30b of the resistive element 30 within the potentiometer 26. Because the resistive element 30 is connected between the positive and negative leads of a DC voltage source, the motion of the wiper 28 causes the potentiometer 26 to output a DC voltage level corresponding to the linear position of the pedal leg 20. The output DC voltage level is input to an electronic accelerator and is used to control the speed of the vehicle.
The relationship between pedal position and vehicle speed is relatively imprecise with the acceleration control mechanism of FIGS. 1A and 1B. The linear position encoder 40 of FIG. 2 attempts to solve this problem. The linear position encoder 40 attaches directly to the accelerator pedal (not shown) and outputs a digital signal that indicates the position of the pedal. A shaft 42 extending from the linear position encoder 40 connects at one end to the pedal, enters the housing 44 of the encoder 40 and connects at the other end to a slidable body 46 within the encoder 40. Several vanes 48 extend perpendicularly away from the body 46 along linear tracks 50. Optical switches 52 mounted to the housing 44 also lie along linear tracks 50. Each optical switch includes a signal generating source (e.g., a light emitting diode, or LED) 56 and a signal receiver (e.g., a photo-transistor) 58 separated by a passageway 54.
In operation, the LED 56 generates an infrared signal which is detected by the photo-transistor 58. As long as the infrared signal emitted by the LED 56 is detected by the photo-transistor 58, the photo-transistor 58 conducts in saturation. When the pedal is depressed and released, however, the slidable body 46 moves through the housing 44, causing the vanes 48 to move through the passageways 54 in the optical switches 52. As the vanes 48 protrude into the passageways 54, the infrared signal from the LED 56 is interrupted and thus is not received at the photo-transistor 58, thus causing the photo-transistor to cease conducting.
As shown in FIG. 3, the collector 58a of each photo-transistor 58 is tied to the high voltage lead 60a of a power supply 60. When the photo-transistor 58 receives the infrared signal and conducts in saturation, the emitter 58b is pulled to a high voltage, as is one input 62a of a logic AND gate 62. Because input 62b of the AND gate 62 is always tied to the high voltage lead 60a of the power supply 60, the AND gate 62 outputs a high voltage that drives the base of output transistor 64. The emitter 64a of the output transistor 64 is tied to the low voltage lead 60b of the power supply 60, so when the transistor 64 conducts in saturation, the collector 64b, and the thus the corresponding output bit of the linear position encoder, is pulled to a low voltage. Therefore, uninterrupted signal detection in the optical switch 52 produces a low voltage output signal.
When the passageway between the LED 56 and the photo-transistor 58 is blocked and the photo-transistor 58 ceases to conduct, a pull-down resistor 66 pulls input lead 62a to a low voltage, thereby pulling the output of the AND gate 62 low and turning off the output transistor 64. As a result, the output collector 64b, which is normally tied to a pull-up resistor (not shown), produces a high voltage at the corresponding output 67 of the linear position encoder 40.
The linear position encoder 40 of FIGS. 2 and 3 has four optical switches 52 and thus has four output transistors 64. Because each transistor 64 outputs either a high or a low voltage, the output of the linear position encoder 40 is a 4-bit digital signal. The pattern of bits in the digital signal depends upon the position of the vanes 48 in relation to the optical switches 52. When the accelerator pedal is fully released and the shaft 42 of the encoder 40 is fully extended, the slidable body 46 abuts the end 68 of the encoder 40 near the accelerator pedal. When the pedal is in this position, a vane 48 along each of the linear tracks 50 protrudes into the passageway 54 of the corresponding optical switch, so none of the switches 52 conducts. The resulting output from the encoder is a digital signal in which all bits are high.
As the pedal is depressed, the vanes begin to exit and reenter the passageways 54 in such a manner that the optical switches 52 cease and then resume conducting in a predetermined pattern. As a result, the bits in the digital output of the encoder 40 change as the pedal is depressed, as the slidable body 46 moves through the housing 44, and as the vanes 48 move through the passageways 54 of the optical switches 52.
The pattern of vanes across the linear tracks 50 and within each linear track 50 is such that only one optical switch 52 changes its conductive state at any given time. In other words, two optical switches cannot begin to conduct or cease to conduct at the same time, nor can one optical switch 52 begin to conduct at the same time another optical switch 52 ceases to conduct. Such a vane pattern enables the encoder 40 to output a digital signal that follows a gray code, i.e., one in which only one bit changes at a time. Gray codes are well-known in the art and will not be described herein. Additional description of the linear position encoder of FIGS. 2 and 3 is provided in Appendix A.
While the accelerator control mechanism of FIGS. 2 and 3 generates more precise acceleration control output, it is not compatible with analog accelerators. This characteristic is particularly problematic because analog accelerators are less expensive and more widely available than the digital accelerators that must be used with the linear position encoder. An example of a widely used analog accelerator is Spectrol Pot Model No. 961-1.
The linear position encoder also presents a problem when one of the output bits, either through failure of an optical switch or the corresponding output circuit, becomes an open-circuit. When this happens, the digital accelerator either shuts the vehicle down permanently or controls the speed of the vehicle in a manner unrelated to the position of the pedal. The former is undesirable because the vehicle becomes useless until the problem is found, and the latter is undesirable because the accelerator control mechanism becomes unpredictable and endangers the operator's safety.